Quietula guaymasiae and Quietula y-cauda occur sympatrically in the Gulf of California (Mexico). Both species are similar in their meristic values but differ in the topography of the lateral line system: the pattern of free neuromasts, their total number and their number in most rows. Quietula guaymasiae is generally somewhat larger than Quietula y-cauda but differences in the number of neuromasts between both species are not size dependent, and the neuromast pattern does not vary between juveniles and adults of either species. The lateral line system of Quietula guaymasiae and Quietula y-cauda is described in detail. For identification of the neuromast pattern and for the comparability of this feature with other gobiids, a nomenclature of the series and the rows of free neuromasts is given.